A Sky News reporter hacked into emails written to Anne Darwin by her husband, John, who faked his death in a canoeing incident in 2002. Photograph: PA

Sky News has admitted that one of its senior executives authorised a journalist to conduct email hacking on two separate occasions that it said were "in the public interest" – even though intercepting emails is a prima facie breach of the Computer Misuse Act, to which there is no such defence written in law.

Gerard Tubb, the broadcaster's northern England correspondent, accessed emails belonging to John Darwin, the "canoe man" accused of faking his own death, when his wife, Anne, was due to stand trial for deception in July 2008. The reporter built up a database of emails that he believed would help defeat Anne Darwin's defence; her husband had pleaded guilty to seven charges of deception before her trial.

The same reporter accessed the email accounts of a suspected paedophile and his wife in an investigation that did not lead to any material being published or broadcast, according to a statement sent to the Guardian by Sky, which is part-owned by Rupert Murdoch's News Corporation.

Both instances of hacking were approved by Simon Cole, the managing editor of Sky News.

John Ryley, the head of Sky News, said the broadcaster had "authorised a journalist to access the emails of individuals suspected of criminal activity" and the hacking in both cases was "justified and in the public interest". Ryley said the broadcaster's decisions required "finely balanced judgment" and they were "subjected to the proper editorial controls".

The broadcaster said it stood by Tubb and that there were instances when the broadcaster believed breaking the law was justified to produce a news story of public interest. It cited the example of a Sky News journalist buying an Uzi machine gun in the UK.

Darwin faked his own death in 2002, "going missing" after he was last seen paddling out to sea in a canoe. He secretly flew to Panama, where he was later joined by his wife, only to return to Britain in 2007.

Walking into a London police station in December 2007, he declared: "I think I may be a missing person", but later that month both he and his wife were charged with fraud after it emerged that they had been photographed in Panama with an estate agent and that Anne Darwin had cashed in her husband's life insurance policy.

He pleaded guilty to seven charges of deception and a passport offence in March 2008, leaving his wife to face six charges of deception and nine of money laundering at a trial due to begin four months later.

At around this point, Sky News said, Tubb discovered that John Darwin used the identity of a friend, John Jones. According to the broadcaster, Tubb conducted an internet search to reveal a Yahoo email account in the name of John Jones and, in the belief that Yahoo accounts were "notoriously weak at the time", the journalist was confident he could gain access with his existing background knowledge. He then sought permission to access the emails, an investigation that led him on to further email accounts.

In the first week of July 2008, Sky News said executives met Cleveland police officials and handed over "pertinent" emails. Anne Darwin was found guilty in the trial that followed shortly afterwards, and was sentenced to six and half years in prison; John Darwin was sentenced to six years, three months.

Shortly after, Tubb produced a story for Sky's news channel and website in which he quoted from emails that had been written by John Darwin to his wife and to a lawyer. A web story, still on Sky News's site at the time of writing, said the channel "has uncovered documentary evidence" that demonstrates "conclusively why John Darwin came back to Britain".

Making only a minimal effort to hide the basis of the story, Tubb's report said Sky News had "discovered an email" from John to Anne dated 31 May 2007, in which he says changes to visa regulations meant he could no longer stay in Panama, where he was hiding on a tourist visa. The report cited evidence from several emails between the couple, including a "final email" from Anne that was not, "as suggested in court", evidence of a "massive row" between them, an email that Tubb said had been "handed to the police by Sky News".

The broadcaster also published a voicemail message on its website, dated 19 May 2007, in which Anne Darwin is clearly heard leaving a message for her husband. The voicemail, part of an interactive graphic, ends with her saying "I'll try and catch you tomorrow. Love you," which the broadcaster said showed "she was doing as much of the running as he was".

Sky News said this was not obtained by phone hacking and a spokesman said: "All of the material obtained by Sky News was via the Darwins' computer-based email accounts. As we have said previously, Cleveland Police were made aware of the source of the material when Sky News shared it with them. We stand by our editorial decisions, which were justified in the public interest."

Intercepting emails is an offence under the Computer Misuse Act, and there is no public interest defence written in law. Theoretically, however, any email hacking charges would have to be brought at the discretion of the police and the Crown Prosecution Service, which could weigh up whether any intrusions could be justified. The role of the CPS in this area is untested, and Keir Starmer, the director of public prosecutions, told the Leveson inquiry in February that he intended to issue guidance to clarify the issue.

Danvers Baillieu, a specialist internet lawyer with Pinsent Masons, said that while there was no public interest defence "it doesn't mean that a jury would convict a person, or a judge would punish them, because there is usually a discretion in such cases". However, he added that "the difficulty for news organisations is the question of where do you draw the line: would it be legitimate to break into somebody's house who is suspected of committing a crime? The issue with computer offences is that people can do it from their offices, and believe it is a lesser offence than any other type of intrusion."

Sensitivities at Sky News are running high because the broadcaster's parent, BSkyB, is subject to a "fit and proper" investigation being conducted by the communications regulator, Ofcom, in the wake of the News of the World phone-hacking scandal. However, that investigation is focused on News Corporation's shareholding, and the continuining directorship of James Murdoch, who stepped down as chairman on Wednesday and who was executive chairman of News International.

Cleveland police said the force did inquire about the provenance of the emails at the time, and said it continued to do so. A statement said: "Cleveland police has conducted an initial review into these matters and can confirm that enquiries are ongoing into how the emails were obtained."

Tubb declined to speak to the Guardian. Cole is on leave, and forwarded inquiries to the Sky News press office.

Ryley said Sky News had asked the law firm Herbert Smith to conduct a separate review of staff email records and payment records in the light of "heightened interest in editorial practices". However, the broadcaster said that because Tubb's email hacking had been sanctioned, his work had not come up as part of that exercise.

Ryley said there were "no grounds for concern" regarding any of its other journalists, and that Sky News believed there were rare occasions when tensions could arise between the law and responsible investigative journalism.

Tubb's authorised email hacking contrasts with another example of a potentially illegal email access, conducted by Patrick Foster while he was employed by the Times. Foster accessed emails belonging to the anonymous police blogger Nightjack to out him as a serving Lancashire police officer, Richard Horton, but his actions were not authorised by any executive.

A story naming Horton was later published by the Times, but the editor, James Harding, said he was not made aware of the unauthorised email access until after the newspaper had begun a court battle to allow the police officer to be named, which it won. Harding said if he had been aware of the hacking previously he would have disciplined the journalist and told him to drop the story. "I squarely do not approve of what happened," the editor told the Leveson inquiry in February.